The data obtained during ATOC showed that acoustic ray arrivals may be resolved and identified to at least 5-Mm range so that these data can be used for acoustic thermometry. The estimated uncertainties in the range- and depth-averaged temperature measurements were about 10 m°C. The time series obtained using the California acoustic source showed a clear annual cycle whose amplitude was similar to that derived from climatology (World Ocean Atlas 94: Levitus et al., 1994; Levitus and Boyer, 1994) and from XBTs of opportunity, yet smaller than the amplitude of the annual cycle derived from TOPEX/POSEIDON altimetry (The ATOC Consortium, 1998). The time series obtained using the Kauai acoustic source is of similar quality in its ability to measure the thermal variability, but it shows greater variability at 100-day timescales. Since ATOC, research has shifted from establishing the integrity of the acoustical measurements (Dushaw et al., 1999; Dushaw, 1999; Worcester et al., 1999) to employing the data oceanographically. One original and continuing goal of the thermometry program is to use the line-integral data to detect patterns of climate variability such as the Pacific Decadal Oscillation (PDO).
Tomographic measurements can be made without risk of calibration drift; these measurements have the accuracy and precision required for large-scale ocean climate observation. Tomographic measurements directly probe the existence and nature of signals at the lowest wavenumbers. These data are also sensitive to variability almost to the ocean bottom, and thus can detect changes below the depths at which XBT and float data are obtained. The impact of the integral measurements on the quality of ocean estimation using numerical models remains an open question, however. This impact is best assessed when all data types - tomography, ARGO, XBTs, TOPEX, etc., are used. The transbasin acoustic measurements may offer a signal-to-noise capability for ocean climate variability that is not readily attainable by an ensemble of point measurements.
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This is a photo of the 75-Hz acoustic source on the fantail of a ship prior to deployment. These sources have an expected life span of about 10 years.
The Kauai acoustic source began transmitting in late 1997, continuing through Fall 1999. After a hiatus of 2 years while marine mammal permitting issues were sorted out, the Kauai acoustic source resumed transmissions in January 2002. Transmissions are expected to continue for another 5 years at regular 4-day intervals. Timeseries obtained from the Kauai source trasmissions compared to TOPEX/POSEIDON satellite altimetry data and the ECCO ocean model state estimates may be seen below.
Data from NPAL Acoustic Thermometry are almost but not quite freely available for use. There are a number of peculiar issues at the moment that prevent posting of these data here for ready download. Those interested in using these data should contact Peter Worcester (pworcester@ucsd.edu) or Brian Dushaw (dushaw@apl.washington.edu). There should be no problems obtaining the data; at the moment we merely want to keep track of who is using the data, and maintain a dialog with users on how they are using and interpreting the data.
